DE69737574T2 - Process for the preparation of a highly porous membrane of polyvinylidene difluoride - Google Patents

Abstract

The present invention relates to the field synthetic polymeric membrane materials formed from casting polyvinylidene difluoride (PVDF) polymer solutions and/or dispersions. Membranes formed in accordance with the present invention are all highly porous. Both internally isotropic membranes and highly asymmetric PVDF membranes are disclosed. The membranes of the invention are useful in a variety of microfiltration and ultrathin applications.

Description

Background of the invention

1. Field of the invention:

The The present invention relates to the field of synthetic polymers Membrane materials obtained by casting polyvinylidene difluoride (PVDF) polymer solutions and / or -dispersions are formed. Membranes shaped according to the invention are all highly porous. Both internally isotropic membranes and highly asymmetric PVDF membranes will disclosed. The membranes of the invention are for a variety of microfiltration applications suitable.

2. Technical background:

considerable Interest and significant efforts were made in the manufacture invested in PVDF polymer membranes. The basic cause of the interest on PVDF membranes as a filter is that PVDF to oxidizing environments, including Ozone, which is widely used in the sterilization of water, is resistant. PVDF is also against an attack by most mineral and organic acids, aliphatic and aromatic hydrocarbons, alcohols and halogenated solvents resistant. It is present in certain aprotic solvents, such as dimethylformamide, N-methylpyrrolidone, and hot Acetone soluble. Furthermore, PVDF has good physical properties at temperatures in the range of -50 up to 140 ° C.

Grandine made something that many consider the first practical microporous PVDF membrane, as in the U.S. Patent No. 4,203,848 described. The membrane was made by a wet thermal phase inversion process. In the process, PVDF in acetone was dissolved at its boiling point of 55 ° C. After casting, the membrane was quenched both thermally and in a water / acetone (20/80 by volume) quench bath. Acetone is a good solvent for PVDF at 55 ° C, but a bad one at room temperature, so Grandine did indeed use a combination of thermal and liquid quenching.

Benzinger poured PVDF ultrafilter from formulations that preferentially used triethyl phosphate as a solvent and a variety of hydroxy compounds as nonsolvents. Please refer U.S. Patent No. 4,384,047 ,

Josefiak disclosed PVDF as one of several "porous shaped bodies" cast using a thermal quenching process. Please refer U.S. Patent No. 4,666,607 ,

The U.S. Patent No. 4,774,132 by Joffee et al. discloses the preparation of activated and modified PVDF structures. Similarly, Degen et al. in the U.S. Patent No. 5,282,971 PVDF membranes modified to contain quaternary ammonium groups covalently bound to the membrane. In the U.S. Patent No. 5,019,260 by Gsell et al. PVDF filtration media with low protein affinities are disclosed.

Costar Corporation discloses in published PCT application no. WO 93/22034 the manufacture of PVDF membranes that are claimed to have improved flow rates. However, the membranes appear to have a completely isotropic structure that contains a dense array of closely spaced and contacting polymer particles. The pores in the membrane appear to be similar to a sintered metal.

Sasaki et al. disclosed a PVDF formulation in U.S. Pat U.S. Patent No. 4,933,081 and in the GB 22,199,786A , published July 20, 1988 (the "Fuji Patent"). The PVDF formulation of Example 1 had a relatively high polymer concentration (20%) exposed to 60 ° C warm air at 30% relative humidity. In addition, the Fuji formulations included a high concentration of polyvinylpyrrolidone, which is a co-solvent / swelling agent. The surface pores in the Fuji PVDF membrane appear to be between about 0.45 μm and 0.65 μm, and the thickness of the membranes ranges from 100 μm to 110 μm.

In terms of structure, the membranes of the Sasaki patent are all disclosed to have 2 degree of asymmetry. In cross section, the membranes have a microporous surface and a coarsely porous surface. The diameter of the pores increases first along a line from the microporous surface the coarse-pored surface, and achieves a minimum pore size between the microporous surface and the coarse-pored surface of the membrane. Subsequently, the pore sizes increase along a line in the direction of the coarse-pored surface, but the increase, and therefore the asymmetry of the membrane, is not as dramatic as in a classical asymmetric membrane of Wrasidlo (US Pat. U.S. Patent No. 4,629,563 ).

Asymmetric or anisotropic membranes are well known in the art. For example, Wrasidlo reveal in the U.S. Patents Nos. 4,629,563 and 4,774,093 and Zepf in the U.S. Patents Nos. 5,188,734 and 5,171,445 each asymmetric membranes and processes for their preparation. The Wrasidlo patent disclosed the first truly asymmetric microfiltration membrane. Asymmetric, as used in the context of the Wrasidlo patent, refers to membranes that have a progressive change in pore size across the cross-section between the microporous skin and the substructure. This is in contrast to reverse osmosis and most ultrafiltration membranes, which have abrupt discontinuities between a "non-microporous skin" and the membrane structure, and which are also referred to in the art as asymmetric.

The Wrasidlo and Zepf patents each disclose highly asymmetric, integral, with microporous Skin covered Membranes with high flow rates and excellent retention properties. The membranes are generally prepared by a modified "phase inversion" method a metastable liquid Two-phase dispersion of polymer in a solvent / non-solvent system prepared, cast and then with a non-solvent will be contacted. The Zepf patent discloses an improvement over the Wrasidlo patent.

• (a) Verdampfen des Lösungsmittels und des Nichtlösungsmittels (Trockenverfahren);
• (b) Exposition gegenüber einem Nichtlösungsmitteldampf, wie Wasserdampf, der auf der exponierten Oberfläche absorbiert (dampfphaseninduziertes Fällungsverfahren);
• (c) Abschrecken in einer Nichtlösungsmittel-Flüssigkeit, im Allgemeinen Wasser (Nassverfahren); oder
• (d) thermisches Abschrecken eines heißen Films, so dass die Löslichkeit des Polymers plötzlich stark herabgesetzt wird (thermisches Verfahren).

Phase inversion processes generally proceed via the following steps: (i) casting a solution or mixture comprising a correspondingly high molecular weight polymer (high molecular weight polymers), a solvent (solvent), and a non-solvent (non-solvent) into a thin film, tube or a hollow fiber and (ii) precipitating the polymer by one or more of the following mechanisms:

• (a) evaporation of the solvent and non-solvent (dry process);
• (b) exposure to a non-solvent vapor, such as water vapor, which absorbs on the exposed surface (vapor-phase-induced precipitation process);
• (c) quenching in a non-solvent liquid, generally water (wet process); or
• (d) thermally quenching a hot film so that the solubility of the polymer suddenly sharply decreases (thermal process).

Schematically, the phase inversion from solution to gel is as follows:

in the Essentially, SOL 1 is a homogeneous solution, SOL 2 is a dispersion, and the gel is the formed polymer matrix. The event (the events, that triggers SOL 2 formation, depends on the phase inversion method used. In general though turns the triggering Event (events) about polymer solubility in the SOL. In which Wet method is poured SOL 1 and with a non-solvent for the Polymer, which triggers the formation of SOL 2, the subsequently "precipitated" into a gel. When gas-phase induced precipitation method SOL 1 is poured, exposed to a gas atmosphere which is a non-solvent for the Includes polymer, which triggers the formation of SOL 2, the then "precipitated" to a gel. In the thermal Process is poured SOL 1, and the temperature of the cast Film is reduced to produce SOL 2, which is subsequently added to a gel "precipitated". In the dry process SOL 1 is poured and brought into contact with a gas atmosphere, such as air, which allows the evaporation of one or more of the solvents, which causes the formation of SOL 2, the next "precipitated" into a gel.

The Nonsolvent in the pouring paste is opposite the polymer is not always completely inert; in fact it is in the Usually not inert and is often referred to as swelling agent. Both Wrasidlo formulations are, as discussed later, the selection both the type and concentration of non-solvent in that respect important, as it is the main factor in determining whether the casting solution in a phase-separated state exists or not.

In general, the non-solvent is the major pore-forming agent, and its concentration in the casting solution greatly affects the pore size and pore size distribution in the final membrane bran. The polymer concentration also affects the pore size, but not as significantly as the non-solvent. However, it does affect the strength and porosity of the membrane. In addition to the major components in the casting solution or paste, minor ingredients such as surfactants or release agents may be present.

Polysulfone is particularly suitable for the formation of highly asymmetric membranes, particularly in the two-phase formulations of Wrasidlo. These are not homogeneous solutions but consist of two separate phases: one is a solvent-rich, clear solution of low molecular weight polymer at low concentrations (eg 7%); the other is a high-concentration, polymer-rich cloudy colloidal solution of higher molecular weight polymer (eg 17%). The two phases contain the same three components, ie polymer, solvent and non-solvent, but in very different concentrations and molecular weight distributions. Most importantly, the two phases are insoluble in one another and, if allowed to stand, separate. Therefore, the mixture must be kept as a dispersion with constant stirring until it is cast as a film. Essentially, in Wrasidlo formulations, the casting solution is provided in a SOL2 (dispersion) state. Thus, the dispersion serves as a starting point for gelation and not as an intermediate step (above), as follows:

These process change was largely for the higher ones Degree of asymmetry and uniform consistency the Wrasidlo membranes in comparison to the previous technique responsible.

It are the non-solvent and its concentration in the casting mix, which cause the phase separation, and not every non-solvent Fulfills this. The two phases separate when they stand but each phase is quite stable in itself. If the Temperature of the mixture changed becomes, enters a phase transition on. Heat generates more of the clear phase; Cooling has the opposite effect. changes in concentration have the same effect, but there is a critical one Concentration range or a window in which the phase-separated System may exist, as discussed by Wrasidlo. Wrasidlo defines these Region of instability on a phase diagram of thus dispersed polymer / solvent / non-solvent at constant temperature than within the spinodal or between lying spinodal and binodal curve, wherein two macroscopically separate layers exist.

On Reason of the big one hydrophobicity of the polymer and due to the thermodynamically unstable state the casting mixture, where two phases previously exist, one solvent-rich and one polymer-rich (a condition that other systems have to go through if they experience phase inversion learn), the unstable Wrasidlo mixtures fall very fast when they are quenched, leaving at the interface a microporous Skin forms, and they consequently become highly asymmetric membranes develop a structure covering all the membranes of Wrasidlo and Zepf patents is common.

The microporous Skin is the fine-pored side of the membrane, which is the air-solution interface or the quench solution interface while of the casting forms. In the Wrasidlo patent and in this disclosure it goes without saying that the term "skin" is not the relative one thick, almost impermeable Polymer layer, which is present in some membranes. Here is the microporous Skin a relatively thin, porous Surface, the above a microporous one Region of variable thickness. The pores of the underlying microporous Region can the same size or slightly smaller than the skin pores. At an asymmetric Membrane gradually take in the pores of the microporous region the size too, if from the skin to the opposite Area of Lead membrane. The region of gradual pore size increase is sometimes called denotes the asymmetric region, and the opposite Not skin surface The membrane is often referred to as a coarsely porous surface. As a contrast to the coarse-pored surface The skin is sometimes referred to as the microporous surface.

at some formulations and casting conditions can have a "skin-forming" effect on the opposite surface the membrane occur - the Surface, the in contact with the casting carrier is and in the casting process is not directly exposed to moist air or the quenching bath. Where such a layer of "opposite Skin "exists, she is usually relatively thin, typically being less than about 10% of the thickness of the membrane ausmacht-. It is also usually highly porous, although the porous network or tissue in cross-section have a more densely packed appearance can be considered the inner region of the membrane between the two skins, if both are available.

polymer membranes can also from homogeneous polymer solutions to be poured. The composition of these formulations is outside the spinodal / binodal region of the phase diagram of Wrasidlo. Membranes made from homogeneous solutions can be poured also be asymmetrical, although they are usually not so highly symmetrical are like those that are poured out of phase-separated formulations become.

The Wrasidlo membranes are related to the prior art membranes improved flow rates and permselectivity. Such improved flow rates and permselectivity arise from the structure of the membranes.

The Zepf patents disclose improved Wrasidlo type polymer membranes with a much larger number of microporous Skin pores of more consistent size and strong increased Flow rates with reduced flow covariance for each any given pore diameter. The improved Zepf membranes are achieved by modification of the Wrasidlo method, comprising reduced casting and quenching temperatures and reduced environmental exposure between to water and quenching. Zepf continues to teach that reduced casting and quenching temperatures the sensitivity of the membrane formation process to small changes minimized in formulation and in process parameters.

The Wrasidlo patents also disclose the preparation of asymmetric PVDF membranes. Please refer U.S. Patent No. 4,774,039 Example 6, column 12, lines 20-34. However, the PVDF membranes made according to the Wrasidlo patent do not seem to have been microporous membranes.

None the microporous discussed here Prior art PVDF membranes have a highly asymmetric one Structure. Consequently, all previous PVDF membranes are in their Flow rates are limited compared to highly asymmetric membranes. As appreciated by the professionals will, would be it is therefore desirable a microporous one PVDF membrane with high degree of asymmetry and high flow rates provide. It would be Furthermore Cheap, ultrathin microporous To provide PVDF membranes with high flow rates, the same whether isotropic or asymmetric in structure. In addition, it would still be beneficial to process to enable to provide the uniform production of all such membranes.

US 5028337 describes composite membranes comprising a base membrane selected from microfiltration, ultrafiltration and reverse osmosis membranes and a coating thereon comprising at least one polyaromatic polymer and sulfonated poly (haloalkylene) which has been subjected to a crosslinking treatment after coating.

Summary of the invention

Have according to the invention We unexpectedly found that it is possible to get a microporous strong produce asymmetric PVDF membrane that has exceptional flow rates. We also found that it is possible to microporous, internally produce isotropic PVDF membranes which have a lattice in cross-section of fibrous webs of the polymeric material.

According to the invention, there is provided a process for producing a polyvinylidene difluoride membrane comprising the steps of:
Providing a pourable paste comprising about 12 to about 20 weight percent polyvinylidene difluoride and 1 to 30 weight percent of a hydrophilic polymer dissolved in a solvent;
Pouring the paste to form a thin film, wherein the temperature of the casting solution is below 21 ° C or above 35 ° C;
Exposing humid air to the thin film for a period of 5 seconds to 120 seconds, the humid air having a relative humidity of 60% to 100%;
Coagulating the film in a water bath, wherein a temperature of the water bath is 40 ° C to 70 ° C; and
Obtaining a formed microporous polyvinylidene difluoride polymer membrane, the membrane comprising a microporous minimum pore surface, an opposite maximum pore surface, and a porous support formed from fibrous webs of polymeric material, the porous support having an isotropic area over at least about 80% of the thickness of the membrane having.

The porous support structure The membranes of this aspect of the invention can be made from a gradient of gradually increasing pore sizes from the microporous surface to the consist of opposite surface.

at In a third aspect of the invention, the PVDF membrane has a microporous surface with minimal pores, one opposite surface with maximum pores, and the space between the surfaces is a porous support formed of fibrous webs of polymeric material. The Membranes can an isotropic structure by at least about 80% of the thickness of the Membrane, and the thickness may be less than about 75 microns, and preferably less than about 30 μm be.

One Fourth aspect of the invention provides a method of manufacture a PVDF membrane, by providing a pouring paste with between about 12 and 20 wt% PVDF and between about 0 and 30 wt% of a hydrophilic polymer, such as PVP, dissolved in a solvent, to water the paste to form a thin film, Exposing the thin film across from a gaseous Environment, coagulating the film in a water bath and winning a shaped microporous PVDF polymer membrane, ready. The produced by this method Membrane can be a microporous surface with minimal pores, one opposite surface having maximum pores and a porous support between the surfaces, wherein the porous one carrier is formed from fibrous webs of polymeric material. In which Method may be the pourable paste mixture while the casting step a temperature between about 21 ° C and about 35 ° C have, and the gaseous Environment may have a relative water humidity between about 50 % and 100%. The exposure step may take a duration of about 2 seconds to about 120 seconds, and the cooling water bath may have a temperature between about 20 ° C and 80 ° C. The PVP in the casting solution mixture may have an average molecular weight of about 45,000 daltons. On the casting and Quenching may involve contacting the shaped membrane a wetting agent such as HPC and / or a surfactant Agent, follow to make the membrane hydrophilic. The procedure can also be a lamination step of the formed membrane to a polymeric Include tissue or a polymeric nonwoven backing.

The Invention provides at a fifth Aspect of an asymmetric integral microporous PVDF polymer membrane with a microporous one surface with minimal pores, one opposite Surface with maximum pores and with an area between the surfaces ready. This area can be a porous one carrier which is formed from fibrous webs of polymeric material, though the porous one carrier Have flow channels can, whose diameter gradually along a gradient of the microporous surface to the opposite surface increases.

One Sixth aspect of the invention is an internally isotropic, microporous PVDF polymer membrane having a microporous surface with minimal pores of a given mean diameter, one first average diameter, and with one opposite surface with maximum pores of a different given middle Diameter. The space between the membrane surfaces can a porous one carrier which is formed from fibrous webs of polymeric material, and can flow channels with having relatively constant diameters. Thus, with this Aspect of the invention, the membrane have three areas. An area near the microporous surface can account for less than 10% of the total thickness of the membrane and can be a pore size accordingly the size of the skin pores on the microporous Side have. Another area may be near the opposite surface may be less than 10% of the total thickness of the membrane and can be a pore size accordingly have those of the pores on the opposite side. The central area of the membrane can be up to 80% or more

Brief description of the drawings

1 is a series of scanning electron micrographs (SEMs) showing cross-sectional views of membranes for comparison purposes:

1a is an internally isotropic PVDF membrane (500X) according to the invention; 1b is an asymmetric PVDF membrane (1,000X) according to the invention; and the 1c and 1d are copies of SEMs from the Costar World patent application no. WO 93/22034 showing the cross-sectional structures of their PVDF membranes (3.000X).

2 is a series of SEMs showing different views of an internally isotropic membrane according to the invention. 2a is a cross-sectional view (500X); 2 B is a microporous surface (1,500X) view; 2c is a view of the coarse-pored surface (500X). The membrane has a microporous surface pore size of about 0.5 μm.

3 is a series of SEMs that have different views of another internal invention shows isotropic membrane. 3a is a cross-sectional view (750X); 3b is a microporous surface (1,500X) view; 3c is a view of the coarse-pored surface (1.000X); the membrane has microporous surface pores of about 2 μm.

4 is a pair of SEMs showing views of an asymmetric membrane according to the invention. 4a is a cross-sectional view (1,000X); 4b is a microporous surface (5,000X) view. The membrane has microporous surface pores of about 0.45 μm.

Full Description of the preferred embodiment

As mentioned above, The present invention relates to our unexpected discovery that it is possible is, microporous PVDF membranes with exceptional Flow rates in 2 distinct forms: internally isotropic and highly asymmetric. The commonality between the two distinct membrane structures lies in the porous Structure, obviously, by the way in which the Polymer goes into the gel state, is formed. This commonality is not difficult when reviewing cross-sectional SEMs of membranes according to the invention compared with state-of-the-art membranes.

Referring now to 1 For example, 4 cross-sectional SEM views are provided for comparison purposes: 1a is an internally isotropic PVDF membrane (500X) according to the invention; 1b is an asymmetric PVDF membrane (1,000X) according to the invention; and the 1c and 1d are copies of the SEMs from the Costar World patent application no. WO 93/22034 showing the cross-sectional structures of their PVDF membranes (3.000X). As can be seen, the internally isotropic membrane of the invention has ( 1a ) relatively fibrous webs of polymeric material. Likewise, the asymmetric membrane according to the invention ( 1b ), from the microporous surface (or just below) to a point about 3/4 of the way to the open-pored surface, where the structure contains distinctly interconnected fiber bundles or webs of polymeric material. In contrast, the Costar membrane ( 1c and 1d Apparently no fiber bundle or tissue structure that can be described as a fibrous tissue. Instead, the polymer appears to be solidified in a spherical or particulate manner corresponding to a sintered metal structure.

One additional common feature of the internally isotropic produced according to the invention and the highly asymmetric membrane is common, is the fact Both types of membranes have a narrow pore area in or adjacent to the microporous surface. That means, so to speak, that the membranes of the invention have a more open substructure than the region or region in or around the microporous Surface. This structure leaves the membranes of the invention essential less resistance to having a fluid flow, when the coarse pores with the incoming inflow be brought into contact. The selectivity of the membrane is limited Pores linked in or adjacent to the microporous surface.

As As used herein, the term "microporous" applies to membranes having microporous membrane surfaces average pore diameters greater than about 0.01 μm. The maximum pore diameter in the microporous surface preferably exceeds about 8 μm not. As is known, membranes with average pore diameters, which are less than 0.01 μm, typically as ultrafiltration, reverse osmosis and gas separation membranes classified.

"Internally isotropic" as used herein refers to membranes that have either a substantially uniform pore size diameter by the mass of the cross-section of the membrane, or on Membranes that do not have significant pore size gradients from the mirco - porous surface opposite area demonstrate. For example, an internally isotropic membrane may have relatively smaller pores in the microporous surface and relatively larger pores in the porous Have polymer substructure, but the pore sizes usually take not gradually from the microporous surface into the polymer substructure. Instead, the microporous surface is a relatively thinner, denser Area, which is a higher Pore density per unit area owns as the porous one Substructure, which is a relatively thicker, less dense area with a lower pore density. Therefore can internal according to the invention Isotropic membranes undergo a relatively abrupt transition in pore size the microporous surface and the opposite surface show what contrary to the gradient of gradually increasing Pore sizes of the microporous Side to the opposite Page stands, as he noted in the asymmetric Wrasidlo membranes becomes.

There the membranes of the invention also a relatively dense skin area near the opposite surface can have the membrane, is it for an internally isotropic membrane possible, Surface pore diameter on both surfaces to show that are very similar to those of a highly asymmetric membrane. Thus, based on the surface pore measurements alone It can not be concluded that a membrane according to the invention internally isotropic or highly asymmetric. The key feature is the internal area: usually at least about the middle 80 % of the thickness of the membrane. In highly asymmetric membranes this is internal area through pores or flow channels of gradually increasing diameter characterized. In contrast, the internal range of internally isotropic membranes pores or flow channels of im Substantially constant diameter. The structural difference is generally very unique in the SEMs and is usually also from the measured values of the flow velocities of membranes with the different internal structures obviously. We have Here is a method for reproducible control and manipulation found and disclosed this internal membrane structure.

Of the Term "asymmetric" as used herein relates to a membrane with a pore size gradient. That is, asymmetric Membranes have their smallest or finest pores in or adjacent to the microporous surface on. The increase in pore size between the microporous surface and the opposite surface the membrane usually runs gradually, wherein the smallest pore size is closest to the microporous surface and the largest pores at or adjacent to the opposite coarse-pored surface being found.

"Integral" as used here refers to membranes made from a single polymer solution or paste are poured. This is in contrast to non-integral or Composite membranes consisting of more than one polymer solution or Paste are poured to form a layered or composite membrane. Composite membranes can even after casting from two or more completely trained membranes are joined together.

Produced according to the invention Membranes are preferably integral, microporous membranes with microporous surfaces Pore sizes in the range of 0.01 μm up to 8.0 μm. Here are examples of membranes with microporous surface pores of 0.1, 0.3, 0.45, 0.5, 0.667, 0.8, 1.0, 2.0, 3.0, and 5.0 μm. The membranes also have an opposite surface. The microporous surface is usually the surface, exposed directly to the environment or the quench bath during casting is. The opposite surface is generally the surface, which is thus not exposed - the Surface, those with the support structure, on which the membrane is poured, is in contact. Accordingly will be the opposite surface sometimes referred to herein as the casting surface.

The opposing surface the invention produced Contains membranes also microporous Pores that are often larger in pore size can as the pores on the microporous Surface. preferred Membranes have opposite ones Surface pore sizes of about 0.05 to about 50 microns. Here are examples of Membranes provided, the opposite surface pore sizes of about 3, 4, 5, 6, 20 and 30 microns have.

Between the microporous surface and the opposite surface have inventively prepared Membranes a porous Substructure. The porous one Substructure can be either substantially isotropic or asymmetric be. The type of structure obtained depends on several factors, including: type and concentration of the polymer, solvents and non-solvents, Casting conditions, like Doctor blade gap and casting solution temperature; Environmental factors, such as exposure time between casting and quenching, and humidity of the exposure atmosphere; and composition and Temperature of the quench bath.

A feature common to both the internally isotropic and the highly asymmetric membranes of the present invention is that preferred membranes have fibrous webs of polymeric material. Please refer 1a and 1b , In contrast, as discussed above, membranes made by the prior art process, such as the Costar membrane, have ( 1c and 1d ), apparently no fibrous structure of polymeric material. Instead, the polymer appears to have solidified in a spherical or particulate manner corresponding to a sintered metal structure.

As mentioned above, the membranes of the present invention have certain pore size differences between the microporous surface and the coarsely porous surface. However, certain membranes are predominantly isotropic in the substructure, while others in the substructure are predominantly are asymmetric. In other words, a membrane of the invention has limited pore sizes in or adjacent to the microporous surface, relative to the substructure of the membrane. However, the thickness of the "skin" area on both surfaces is relatively thin: usually less than about 10% and preferably less than about 7% of the total thickness of the membrane. The limited pore area in or adjacent to the microporous surface allows the membranes of the invention to be very open in the substructure, allowing for good flow while maintaining good retention properties due to the restricted pore area.

The produced according to the invention PVDF membranes, whether highly asymmetric or internally isotropic, are by a relatively large one Hollow volume marked. In some embodiments, this is the void volume at least 45%, 50% or 55%. Other embodiments of the membranes of the invention have hollow volumes of at least 60%, 65% or 70%. Again other embodiments Hollow volumes of at least 75%, 80% or 85% up. The void volume can be determined by comparing the weight of a dry and a wet membrane or by calculating the density of a dry membrane and comparison with the density of a massive mass of the polymer or a mixture of polymers of equal volume be determined.

The membranes of the present invention are generally very "open" membranes that provide limited resistance to fluid flow and yet are capable of effectively separating particles based on principles of tortuosity and minute pore sizes. Accordingly, the membranes of the present invention, desirably, have exceptional flow rates. For example, the following table lists the pore size of the microporous surface, the pore size of the facing surface, and the flow rate. TABLE I internal structure microporous surface pore size (μm) opposite surface pore size (μm) Flow rate (cm / min at 10 psid) 68.95 kPad isotropic 0.1 3.0 7 asymmetrical 0.1 3.0 40 Isotrop 0.3 3.0 31 asymmetrical 0.45 - 79 asymmetrical 0.5 20.0 73 asymmetrical 0,667 - 81 asymmetrical 0.8 20.0 84 isotropic 1.0 4.0 36 isotropic 2.0 5.0 53 asymmetrical 2.0 30.0 144 asymmetrical 2.0 20.0 178 asymmetrical 2.0 20.0 268 isotropic 3.0 5.0 72

• Anmerkung: In der vorhergehenden Tabelle sind sämtliche Porengrößen in μm angegeben, und sämtliche Fließgeschwindigkeiten sind in cm/min bei 10 psid (68,95 kPad) angegeben.

It is believed that membranes of the present invention have significantly improved flow rates over prior art membranes. For example, the Costar patent compares Costar membranes with PVDF membranes commercially available from Millipore Corporation. See Costar Table 1, page 11. The following table compares the flow rates of membranes of the invention with Costar and Millipore membranes. Table II microporous surface pore size Flow rate, according to the invention mean flow pore size, according to the invention Costar patent flow rate Millipore flow rate Millipore mean flow pore size 0.1 7-40 - 3.6 to 4 2.9 0.228 0.22 - - 8-10 6.5 0.424 0.3 31 - - - - 0.45 79 - 20-60 16 - 0.5 73 0.85 - - - 0.65 - - 40-50 38 1.15 0,667 81 - - - - 2.0 144-268 1.9 125-190 - -

• Note: In the previous table, all pore sizes are in microns, and all flow rates are in cm / min at 10 psid (68.95 kPad).

To For comparison purposes, the pore size is medium flow, where available, specified. It is important to note that the average flow pore sizes for the membranes of the invention closer to the actual Skin pore sizes are as it is for the tested Millipore membranes is the case. To the differences in the flow rate Evaluating between different membranes is typically done a comparison of membranes with similar ones Skin pore sizes. Indeed the more meaningful Comparison between membranes with similar mean flow pore sizes. One such comparison shows the drastic improvement in flow rate for the membranes according to the invention Compared to the available standing state of the art membranes. Membranes made according to the invention were, are relatively thin. For example, you can according to the invention, internally isotropic membranes so thin such as about 20 to 25 microns are prepared and are preferably about 25 to about 50 microns thick. Produced according to the invention Asymmetrical membranes are generally slightly thicker than their corresponding ones internally isotropic. For example, preferred asymmatic membranes between about 60 and about 125 microns thick.

Manufactured according to the invention Membranes are made with relatively high polymer concentrations of about 12 produced up to about 20% or more. The temperature of the paste is preferably between 21 ° C and 35 ° C for high asymmetric membranes and either less than 21 ° C or more as 35 ° C for internal isotropic membranes. Furthermore is in preferred embodiments after the pouring the polymeric paste or the polymer film the film of moist air exposed. The exposure times are preferably relatively long, from 5 or 10 seconds to 1 or 2 minutes or longer; the Moisture is preferably relatively high, from 60% to 100% relative Humidity. As you can see, you can higher Humidities or longer Exposure times are used, with a generally corresponding or proportional decrease in each other to comparable To get results. After exposure, the film is placed in a quench bath, the relatively high levels of non-solvent for the polymer contains deterred. The quenching temperature is preferably between 45 ° C and 70 ° C.

The above method produces PVDF membranes with exceptional flow characteristics. In fact, we have made membranes that have flow rates comparable to asymmetric polysulfone membranes and that have bubble points as low as 5 pounds per inch ² differential (psid) (34.47 kPad). Pounds per inch ² differential (kPad) is the difference in pressure that exists on the opposite sides of the membrane; the bubble point of the membrane correlates with the permeability of the membrane, with a low bubble point indicating that a relatively small pressure differential is required to force air through a previously wetted membrane. Bubble points of the PVDF membranes of the invention are comparable to the bubble points of highly asymmetric Wrasidlo sulfone polymer membranes with similar average flow pore sizes. This is not the case with the prior art PVDF membranes, indicating that the membranes of the present invention have porosity and flow rates superior to prior art PVDF membranes.

Higher polymer concentrations in the formulation according to the invention confer strength and improve the elongation properties of the membranes. In addition, we note a reduction in macro-voiding in the substructure. The exposure of the cast film to humid air appears to be responsible for the generation of the open pores in a manner consistent with the work we have done with sulfone polymers and those in the copending application U.S. Patent Application Serial No. 08 / 206.114 , filed March 4, 1994, published as WO 95/23640 is described.

Interestingly, However, we observe that damp air exposure in conjunction with the production of the present membranes to a clear different structure leads, as those obtained using sulfone polymers becomes. Sulfone polymer membranes that have a similar environmental exposure are cast, have a two-layer structure: a microporous surface that over one isotropic area that over an asymmetric area. It seems that the isotropic area the membranes of the invention generated or at least triggered is characterized by in the "gas phase induced Precipitation process "occurring interaction between the water vapor in the air and the polymer film, the homogeneous or triggers isotropic pore formation. This is analogous to cellulose mixed ester or cellulose nitrate membranes. However, it seems that negligible evaporation of solvent or non-solvents so that in the quench bath the quench liquid penetrates into the isotropic region and fixes it and the asymmetric one Area created and strengthened.

In addition, with sulfone polymer membranes made according to this process, in certain instances we find an obvious limitation on the pore size distribution below the microporous skin. Such a structure would aid in explaining the denser pores found in porosimetric analyzes, such as our observation of a membrane with a maximum pore size of 1.0 μm and an average pore size of 0.8 μm. This may be evidence of a process of internal "skinning" similar to skin formation in the Wrasidlo and Zepf membranes. Support for this opportunity is provided by Michaels in the U.S. Patent No. 3,615,024 , Col. 5, lines 43-54, where it is disclosed that a gradient pore structure occurs when water permeation into a cast film is limited by a sealed skin initially formed by the water. Or it is alternatively possible that, although the membranes in the isotropic region are apparently isotropic on visual inspection, they actually have a pore distribution which accounts for the porosimetry data and the higher bubble point than would be expected given the large pore sizes.

Preferably become membranes according to the invention poured a lot of water, which is strictly a non-solvent is included. By coordination of solids concentration, moisture and exposure time, we synthesized highly porous membranes, which have sufficient strength to qualify them for applications, the ultrathin Require membranes. In the attached tables and SEM photos we make comparisons for Formulations, pore sizes and other features and conditions. As mentioned, due to the improved Strengths of the membranes of the invention, the PVDF membranes as highly porous ultrathin Membranes without fabric reinforcement be synthesized. However, previous experiments have shown that PVDF for it is suitable to be poured onto tissue, if necessary is looked at. Thus, we according to the invention also certain preferred Laminating and reinforcement techniques ready.

to Improvement of strength and handling characteristics of the membranes of the invention, the Membranes are laminated with or on different tissues. For example Suitable laminating materials include woven and nonwoven materials, meltblown materials, polyesters and polyolefins. As examples are polyester, polypropylene, nylon, polyethylene terephthalate and Polyethylene among many polymers as a fabric or non-woven backing for the membranes of the invention suitable. The choice of the particular laminating material to use takes place depending on the particular application. To membranes of the invention to laminate, as is known to those skilled in the art, heat or adhesive or techniques are used.

In connection with the preparation of commercial quantities of PVDF membranes of the invention, it is to be understood that the casting or production line preferably has a range in which the film is exposed to wet conditions and controlled air flow between the pour point, such as a doctor blade or a slit blade. Coating machine, and is exposed to the point of quenching water. The same applies to the above-mentioned large-pore sulfone polymer membranes. However, due to differences in set and cure times, PVDF often requires much longer air exposure times. Accordingly, either the belt speed must be slowed or the moisture tunnel for PVDF membranes must be extended. For example, if a PVDF membrane is cast at 20 ft per minute (6.10 m / min) and requires 1 minute humid air treatment, the exposure length must be 20 ft (6.10 m) m).

The membranes according to the invention can be hydrophobic or hydrophilic. A membrane that is hydrophobic after casting can be aftertreated with a wetting agent to them to make hydrophilic. Suitable wetting agents include surfactants Medium and hydrophilic polymers. Examples of suitable surface-active Agents are ZONYL (DuPont, Bloomington, DE) and TRITON X-100 (Rohm & Haas, Philadelphia, PA). An example for a hydrophilic polymer is hydroxypropyl cellulose (HPC). A preferred HPC treatment exists in soaking the membrane in a bath containing an aqueous HPC solution; the bath can also be one or several surface active Agent included alone or in combination with HPC. The wetting agents can also be added to the quench bath, so that the membrane as a part the quenching step is rendered hydrophilic.

The membranes according to the invention are for the entire area microporous Filtration applications, including electronic water filtration, where ozone is present. A very important application of membranes according to the invention is in batteries, with a diaphragm as a separator between different cells of a battery or to include an alkaline Gels is used.

These Application requires a highly porous ultrathin membrane, about 30 microns thick, the towards propylene carbonate, Potassium hydroxide (KOH) 1 molar, is resistant. In water would this Concentration a pH of about 14 correspond. Although propylene carbonate is a latent solvent for PVDF at 80 ° C is and something to worry about Propylene carbonate was also available at room temperature, perhaps could be a strong swelling agent, We have found that the PVDF membranes of the invention over a Attack by propylene carbonate even at temperatures slightly above the room temperature are resistant. In fact, the membranes stopped in immersion experiments we have performed on membranes according to the invention, successfully met the stringent conditions of propylene carbonate, the Kalimhydroxid (KOH) 1 molar contains, were given. Further held a sample of a PVDF membrane according to the invention, which on Nylon fleece was cast, also propylene carbonate / KOH stood. In each of these immersion experiments showed inventively produced Membranes have good chemical resistance without curling Membranes and without obvious weakening of the membranes.

The dope mix the membrane can, in addition to PVDF, a hydrophilic polymer such as polyvinylpyrrolidone or polyethylene glycol. These polymers can increase the viscosity of the slurry mixture increase and can also the porosity and affect pore structure. Such hydrophilic polymers interact Naturally with the other factors that affects the structure of the membrane, like casting temperature, Quenching temperature, exposure time to moist air, temperature and relative humidity of the air to which the membrane is exposed, and the same. The membranes of the invention can without a hydrophilic polymer or with a hydrophilic polymer having a Final concentration in the pourable paste mixture up to about 30%, often at least 1%, 2%, 3%, or 5% of the casting paste mixture (by weight) to be poured.

There many of the PVDF membrane formulations according to the invention have high polymer concentrations and thus have high viscosities, are the formulations according to the invention Spinning microporous Hollow fibers well adapted. In one embodiment for the PVDF hollow-fiber membranes according to the invention is either air or a non-solvent, such as water, the Solvent or other chemicals, such as a non-solvent, than the lumen fluid is used, and the outer surface becomes different environments, like moist air, dry air or other environments, before the Quenching in a liquid, like water, exposed. The resulting membrane points to the outside The membrane denser pores, as solvents and non-solvents possibly outward migrated as external ambient water wandered inside. We have found that the viscosity of the formulations of the invention can be made high enough to the forming hollow fiber membrane while their fall through the gap of moist air intact.

For PVDF there it's multiple sources, and it's based on the average molecular weight, available in several classes. We have determined that HYLAR-461 (available from Ausimont Co., Morristown, NJ) and KYNAR-761 (available from Atochem Co., Philadelphia, PA) for casting the membranes of the invention is particularly suitable. The preferred, produced according to the invention Membranes and manufacturing processes are now covered by the following Examples and with reference to the accompanying figures.

EXAMPLE I

Effect of exposure duration

We put a pouring paste containing 15.9% by weight PVDF polymer HYLAR-461, 0.9% by weight lithium chloride, 3.7% by weight of water, 2.3% by weight of polyvinylpyrrolidone (PVP K-17 MW 9,000, available by BASF, Mt. Olive, NJ) and 77.2% by weight of dimethylacetamide (DMAC). as a solvent contained. Several membrane samples were prepared using a squeegee with a 7 mil (178 μm) blade gap on a polyethylene coated conveyor belt Poured paper. Preferably, the casting solution mixture is below about 32 ° C, when it is poured. After casting, the resulting Membranes at 25 to 27 ° C and 100% or 80% relative humidity for different times to air exposed as shown in Table III. Subsequently, the membranes were in Quenched in a water bath at a temperature of about 60 ° C.

After coagulation, the membranes were washed with deionized water and then treated with a 0.1% aqueous hydroxypropylcellulose (HPC) solution for 10 to 15 seconds to make the membranes hydrophilic and then air dried. The recovered membranes had a thickness between 25 microns and 30 microns. The membranes were each tested for water permeability at 10 psid (68.95 kPad) on a 47 mm diameter disk (approximately 35 mm effective diameter, 9.5 cm ² area). The mean flow-through pore sizes of the membranes were determined with a Coulter Porometer and the surfaces and cross-sections were checked by Scanning Electron Microscopy (SEM).

A representative SEM for the membrane of sample 1b is shown in FIGS 2a to 2c provided. As seen from the cross-sectional view of the membrane ( 2a ), the membrane has a porous substructure that is substantially isotropic. However, the pores in the substructure are typically larger than the pores in and adjacent to the microporous surface. The difference in pore sizes, such as between the microporous surface and the coarsely porous surface, is easily detectable when comparing the SEM of the microporous surface ( 2 B ) with the SEM of the coarse-pored surface ( 2c ).

• Anmerkung: Die Wasserdurchflusseinheiten sind ml/min/9,5 cm² bei 10 psid (68,95 kPad)

Exposure conditions, water flow data and pore sizes are shown in Table III. TABLE III Sample ID Exposure time / moisture Water flow mean flow pore size (μm) Surface pore size (open side) Surface pore size (dense side) 1a 8 seconds 100% 377 - 3 μm 0.1 μm 1b 10 seconds 100% 694 0.85 20 μm 0.5 μm 1c 30 seconds 100% 800 1.0 20 μm 0.8 μm 1d 20 seconds 80% 94 - N / A N / A 1e 120 seconds 80% 1364 1.9 30 μm 2.0 μm

• Note: The water flow units are ml / min / 9.5 cm ² at 10 psid (68.95 kPad)

EXAMPLE II Effect of Moisture and solvents

We put a pouring paste containing 16% by weight of PVDF HYLAR-461, 8.0% by weight of water, 3.0% by weight of PVP K-17 and 73 wt .-% N-methylpyrrolidone (NMP) as a solvent contained. 2 membrane samples were washed using a squeegee a blade gap of 7 mils (178 μm) on a conveyor belt poured from polyethylene-coated paper. After pouring were the resulting membranes at 25 to 27 ° C and 70% relative humidity for different Times over Exposed to air, as shown in Table IV. Subsequently were quenched the membranes in a water bath at a temperature of about 60 ° C.

After coagulation, the membranes were washed with deionized water and then treated with a 0.1% aqueous HPC solution to make the membranes hydrophilic and then air dried. The membranes obtained had thicknesses between 25 and 30 microns. The membranes were tested for water permeability at 10 psid (68.95 kPad) on a 47 mm diameter disk. The surfaces and cross-sections of the membranes were checked by SEM. The SEMs of the membranes showed a similar structure to those in the 2a to 2c are shown. However, the cross-sectional SEMs indicated some macrocavity formation. This factor indicates that NMP for PVDF is not as good a solvent as DMAC, although higher polymer concentrations could solve this problem.

• Anmerkung: Die Wasserdurchflusseinheiten sind ml/min/9,5 cm² bei 10 psid (68,9 kPad)

Exposure conditions, water flow data and pore sizes are shown in Table IV. TABLE IV Sample ID Exposure time / moisture Water flow Surface pore size (open side) Surface pore size (narrow side) 2a 10 seconds 70% 66 3 μm 0.1 μm 2 B 120 seconds 70% 344 4 μm 1.0 μm

• Note: The water flow units are ml / min / 9.5 cm ² at 10 psid (68.9 kPad)

EXAMPLE III

Effect of polymer concentration

Regarding the presence of macrocavities in the example II produced membranes led We conducted the following experiments to study the effects of polymer concentration to determine the membrane structure.

We put a pouring paste containing 20% by weight of PVDF HYLAR-461, 5.0% by weight of water, 1.5% by weight of PVP K-17 and 73.5 wt% NMP as solvent contained. Two membrane samples were placed on a conveyor belt of polyethylene coated Paper using a squeegee with a knife gap of 7 mils (178 microns) poured. After casting The resulting membranes were at 25 to 27 ° C and 70% relative humidity for different Times, as shown in Table V, exposed to air. Subsequently, the Quenched membranes in a water bath at a temperature of about 55 ° C.

To In coagulation, the membranes were washed with deionized water and then with a 0.1% aqueous HPC solution treated to make the membranes hydrophilic and then air dried. The recovered membranes possessed Thicknesses between 25 μm and 30 μm. The membranes were grown on a 47 mm diameter disc at 10 psid (68.95 kPad) water permeability tested. The surfaces and cross-sections of the membranes were examined by SEM. The had membranes prepared in this way a virtually identical structure to those in connection with Example I, and did not have the macrocavities, which were found in connection with Example II.

• Anmerkung: Die Wasserdurchflusseinheiten sind ml/min/9,5 cm² bei 10 psid (68,95 kPad)

Exposure conditions, water flow data and pore sizes are shown in Table V. TABLE V Sample ID Exposure time / moisture Water flow Surface pore size (open side) Surface pore size (narrow side) 3a 60 seconds 70% 297 3 μm 0.3 μm 3b 120 seconds 70% 2542 20 μm 2.0 μm

• Note: The water flow units are ml / min / 9.5 cm ² at 10 psid (68.95 kPad)

EXAMPLE IV

Effect of moisture and exposure time

Regarding the difference in the flow rate and pore size in the according to example III produced membranes We do the following experiments to get the effects of moisture and Exposure duration to determine.

We put a pouring paste containing 16% by weight of PVDF HYLAR-461, 8.0% by weight of water, 3.0% by weight of PVP K-17 and 73 wt .-% NMP as a solvent contained. Four membrane samples were made using a squeegee with a 7 mil (178 μm) blade gap on a polyethylene-coated conveyor belt Poured paper. After casting For example, the resulting membranes became more relative at 25 to 27 ° C and 70% or 100% Moisture for different times, as shown in Table VI, to air exposed. Subsequently The membranes were placed in a water bath at a temperature of about 55 ° C deterred.

To In coagulation, the membranes were washed with deionized water and then with a 0.1% aqueous HPC solution treated to make the membranes hydrophilic and then air dried. The recovered membranes possessed Thicknesses between 25 μm and 30 μm. The membranes were run at 10 psid (68.95 kPad) on a 47 mm diameter disk on water permeability tested. The surfaces and cross-sections of the membranes were examined by SEM.

The results of a representative membrane are in the 3a to 3c which represents a series of SEMs of the membrane prepared in conjunction with sample 4c below. 4a shows that the membrane is even more open in cross section than in connection with Example 1 (FIG. 2a ) shown membrane. Again, the membrane has a porous substructure that is substantially isotropic. However, the pores in the substructure are typically larger than the pores in and adjacent to the microporous surface.

The difference in pore sizes, such as between the microporous surface and the coarse-pored surface, is easily detectable when the SEM of the microporous surface (Fig. 3b ) with the SEM of the coarse-pored surface ( 3c ) is compared.

• Anmerkung: Die Wasserdurchflusseinheiten sind ml/min/9,5 cm² bei 10 psid (kPad)

Exposure conditions, water flow data and pore sizes are shown in Table VI. TABLE VI Sample ID Exposure time / moisture Water flow Surface pore size (open side) Surface pore size (narrow side) 4a 10 seconds 70% 66 3 μm 0.1 μm 4b 120 seconds 70% 344 4 μm 1.0 μm 4c 10 seconds 100% 1690 20 μm 2.0 μm 4d 60 seconds 100% 502 5 μm 2.0 μm

• Note: The water flow units are ml / min / 9.5 cm ² at 10 psid (kPad)

A comparison of the properties of membrane 4a with those of membrane 4c shows the drastic difference in the effect of 70% moisture vs. 100% humidity at this temperature. Similarly, a comparison of the properties of membrane 4a with those of membrane 4c shows that increased exposure time can balance somewhat the low humidity. However, membrane 4d showed a reduced flow rate and smaller pore size on the open side of the membrane as compared to the membrane of Example 4c. This shows that at 100% humidity, a membrane may be overexposed, and that the interaction of relative humidity and exposure time is not present in all This is synergistic.

EXAMPLE V

Effect of the solvent

We put a pouring paste containing 13.8% by weight of PVDF HYLAR-461, 6.9% by weight of glycerol, 1.7% by weight. Water, 2.0% by weight of PVP K-17 and 75.6% by weight of dimethylformamide (DMF) as a solvent contained. Two membrane samples were prepared using a squeegee with a 7 mil (178 μm) blade gap on a polyethylene-coated conveyor belt Poured paper. After casting The resulting membranes were at 25 to 27 ° C and 100% relative humidity for different Times over Exposed to air as shown in Table VII. Subsequently were quenched the membranes in a water bath at a temperature of about 60 ° C.

To In coagulation, the membranes were washed with deionized water and then with a 0.1% aqueous HPC solution treated to make the membranes hydrophilic and then air dried. All had resulting membranes Thicknesses between 25 μm and 30 μm. The recovered membranes were placed on a 47 mm diameter disc tested for water permeability at 10 psid (68.95 kPad). The surfaces and Cross sections of the membranes were examined by SEM. By the SEMs observed structures had a similar appearance to those which were shown in connection with Examples I and IV.

• Anmerkung: Die Wasserdurchflusseinheiten sind ml/min/9,5 cm² bei 10 psid (kPad)

Exposure conditions, water flow data and pore sizes are shown in Table VII. TABLE VII Sample ID Exposure time / moisture Water flow Surface pore size (open side) Surface pore size (narrow side) 5a 10 seconds 100% 682 5 μm 3 μm 5b 60 seconds 100% 199 6 μm 5 μm

• Note: The water flow units are ml / min / 9.5 cm ² at 10 psid (kPad)

EXAMPLE VI

Preparation of carrier-supported membranes

We put a pouring paste containing 15.4% by weight of PVDF HYLAR-461, 7.4% by weight of water and 77.2% by weight. DMF as a solvent contained. Two membrane samples were prepared using a squeegee cast with a 7 mil (178 μm) blade gap onto a nylon nonwoven backing. After casting The resulting membranes were at 25 to 27 ° C and 100% relative humidity exposed to air for either 10 seconds or 60 seconds. Subsequently were quenched the membranes in a water bath at a temperature of about 60 ° C.

To In coagulation, the membranes were washed with deionized water and then with a 0.1% aqueous HPC solution treated to make the membranes hydrophilic and then air dried.

The recovered membranes were dissolved in solutions of propylene carbonate containing KOH 1 molar (pH ~ 14). After recovering the membranes from the dip, according to the Following example, there was no sign of deterioration or loss of strength of the membrane. There was no significant change determined by tensile strength and elongation.

EXAMPLE VII

Tensile strength and elongation at break

We put pouring pastes containing 15.4% by weight of PVDF HYLAR-461, 7.4% by weight of water and 77.2% by weight. DMF as a solvent contained. Two membrane samples were prepared using a squeegee with a 7 mil (178 μm) blade gap on a conveyor belt cast from polyethylene-coated paper. After pouring were the resulting membranes at 25 to 27 ° C and 100% relative humidity exposed to air for either 10 seconds or 60 seconds. Subsequently were quenched the membranes in a water bath at a temperature of about 60 ° C.

To Coagulation, the membranes were washed with deionized water and subsequently with a 0.1% aqueous HPC solution treated to make the membranes hydrophilic and then air dried. The recovered membranes were dissolved in solutions of polypropylene carbonate, the KOH 1 molar (pH ~ 14) contained immersed.

After recovering the membranes from the dip, there was no evidence of degradation or loss of membrane strength. There was no significant change in tensile strength and elongation as shown in Table VIII. TABLE VIII Exposure duration (days) Membrane thickness (μm) Tensile strength (g / cm ² ) Elongation at break (%) 1 30 725 77 7 30 700 85.7 14 30 748 69.2 22 30 783 84

EXAMPLE VIII

Production of asymmetric PVDF membranes

We made a highly asymmetric PVDF membrane from a casting paste, the 14.8% by weight of PVDF HYLAR-461, 18.8% by weight of t-amyl alcohol, 4.6% by weight PVP K-30 (average M.W. ~ 45,000), 2.3 wt% water and 59.5% by weight DMF as solvent contained. The sample was coated using a squeegee Squeegee gap of 12 mils (305 μm) on a conveyor belt cast from polyethylene-coated paper. The temperature of the dope mix was always between 21 ° C and 35 ° C held. After casting The resulting membrane was at 25 to 27 ° C and 80% relative humidity 25 seconds over Exposed to the air. Subsequently The membrane was quenched in a water bath at a temperature of about 58 ° C.

To Coagulation, the membrane was washed with deionized water and subsequently with a 0.1% aqueous HPC solution treated to make the membrane hydrophilic. Subsequently, the Membrane at about 70 ° C oven dried and at 10 psid (68.95 kPad) on a 47 mm diameter disk on water permeability tested.

The resulting membrane had a thickness of about 70 μm to 100 μm. Surface and cross section of the membrane were examined by SEM. 4a and 4b show the view of the microporous surface and the cross-sectional view of the membrane. As seen from the cross-sectional view ( 4a ), the membrane has a graded pore structure from the microporous surface in and through a substantial portion of the substructure of the membrane. Such a graded pore structure exhibits classical asymmetric structure in which there is a gradual increase in pore size from the microporous surface into the substructure of the membrane.

Water flow and pore size are shown in Table IX. TABLE IX Samples ID Water flow (ml / min / 9.5 cm ² at 10 psid) (68.95 kPad) Membrane pore size 8th 750 0.45 μm

EXAMPLE IX

Producing an asymmetric PVDF membrane

We made a highly asymmetric PVDF membrane from a casting paste, the 14.6% by weight of PVDF HYLAR-461, 18% by weight of t-amyl alcohol, 4.8% by weight PVP K-30, 2.3% by weight Water and 60.3 wt .-% DMF as solvent contained. The samples were made using a squeegee with a Squeegee gap of 9.5 mil (241.3 μm) on a conveyor belt cast from polyethylene-coated paper. After the casting was the resulting membrane at 25 to 27 ° C and 100% relative humidity for 25 Seconds over air exposed. Subsequently The membrane was placed in a water bath at a temperature of about Quenched 50 ° C.

To Coagulation, the membrane was washed with deionized water and subsequently with a 0.1% aqueous HPC solution treated to make the membrane hydrophilic. After that, the membrane became air dried and at 10 psid (68.95 kPad) on a 47 mm diameter disk on the water permeability tested. The mean flow pore size was also measured using a Coulter Porometer certainly. The resulting membrane had a thickness of between 50 μm and 75 μm.

Water flow and pore size are shown in Table X. TABLE X Samples ID Water flow (ml / min / 9.5 cm ² at 10 psid) (68.95 kPad) Membrane pore size 7 769 0.667 μm

EXAMPLE X Preparation of a hydrophobic PVDF membrane

We set up a PVDF membrane according to the formulation and procedure from example VIII, with the only difference being in the missing one Post-treatment of the membrane with HPC was. The water flow and the membrane pore size were identical to those given in Table IX. Indeed required the membrane to be pre-wetted with a 50% aqueous solution solution of isopropyl alcohol. Other pre-wetting solutions which are hydrophobic for the invention Suitable membranes include glycerol, zonyl, triton, PVP and HPC.

EXAMPLE XI

Preparation of an internally isotropic PVDF membrane

We prepared an internally isotropic PVDF membrane from a slurry formulation similar to that of Example VIII, but we poured the membrane at a casting solution temperature above 35 ° C. 3a shows the membrane in cross section, and the 3b and 3c show the microporous surface or the opposite surface. As seen from the cross-sectional view ( 3a ), the membrane has a substantially constant pore structure in a central region between a dense region adjacent to the microporous surface and a region of the "opposite skin" adjacent to the opposite surface of the membrane. This substantially constant pore structure is a preferred embodiment of the internally isotropic PVDF membrane of the invention.

EXAMPLE XII

Properties of HYLAR-461 and KYNAR-761

A preferred form of PVDF is available under the trade name HYLAR-461. Another preferred form of PVDF is available under the trade name KYNAR-761. The present invention contemplates membranes cast using any suitable form of PVDF. Preferably, the invention uses HYLAR-461, KYNAR-761, or a substantially similar material having properties corresponding to those provided in the following Table XI. TABLE XI property HYLAR461 KYNAR-761 specific weight 1.75 to 1.77 1.78 specific volume 0.56-0.57 cc / g 0.56 cc / g refractive index 1.42 - melting point 311-320 ° F [155-180 ° C] 352 ° F [177,8 ° C] mold shrinkage 2-3% - water absorption 0.04% <0.04% (24 h) tensile strenght 4,000-7,000 psi at 77 ° F [27.58-41.37 MPa at 25 ° C] 2,000-3,000 psi at 212 ° F [13.79-20.68 MPa at 100 ° C] - elongation 4,500-7,500 psi at 77 ° F [31,03-51.71 Mpa @ 25 ° C] 2,500-3,500 psi @ 212 ° F [17,24-24,13 Mpa @ 100 ° C] 7,800 psi [53.78 MPa] thermal conductivity 0.70-0.87 BTU in / h ft ² ° F [0.10-0.13 Watt / m ° C] 1.32 BTU in / h ft ² ° F [0.19 Watt / m ° C] Specific heat 0.30-0.34 BTU / lb / ° F [1256.04-1423.51 J / kg ° C] 0.23 BTU / lb / ° F [962.96 J / kg ° C] melt viscosity 25,500-30,000 poise at 100 / s and 450 ° F [2,550-3,000 Pa-s at 100 / s and 232.2 ° C] -

Claims (18)

Process for the preparation of a polyvinylidene difluoride membrane, comprising the steps: Providing a pouring paste, the about 12 to about 20 wt .-% polyvinylidene difluoride and 1 to 30% by weight of one in a solvent dissolved hydrophilic polymer; Pour the paste to a thin film with the temperature of the dopant being below 21 ° C or above 35 ° C; Let absorb of moist air on the thin Movie for a period of 5 seconds to 120 seconds, with the wet Air has a relative humidity of 60 to 100%; Coagulate of the film in a water bath, wherein a temperature of the water bath 40 to 70 ° C is; and Obtaining a formed microporous polyvinylidene difluoride polymer membrane, wherein the membrane is a microporous surface with minimal pores, one opposite surface with maximum pores and one of fibrous webs of polymeric material formed porous carrier wherein the porous support comprises a isotropic area over at least about 80% of the thickness of the membrane. The method of claim 1, wherein the moist air has a relative humidity of water of 100%. The method of claim 1 or 2, wherein the exposing step 10 seconds to 1 minute lasts. A method according to any one of claims 1 to 3, wherein the hydrophilic polymer is polyvinylpyrrolidone summarizes. The method of claim 5, wherein the polyvinylpyrrolidone has an average molecular weight of about 45,000 daltons. The method of claim 5, wherein the polyvinylpyrrolidone has an average molecular weight of about 9,000 daltons. Method according to one of claims 1 to 6, further comprising the Step of getting in touch comprising the membrane with a wetting agent, whereby the membrane made hydrophilic. The method of claim 7, wherein the wetting agent Hydroxypropylcellulose includes. The method of claim 7, wherein the wetting agent includes a surfactant. Method according to one of claims 1 to 9, wherein the membrane a hollow fiber. A method according to any one of claims 1 to 10, further comprising Step of laminating a substrate on the membrane. The method of claim 11, wherein the substrate comprises a polymer nonwoven fabric. Method according to one of claims 1 to 12, wherein the minimum Pores of the microporous surface a first average diameter and maximum pores the opposite surface have a second average diameter, wherein the relationship of the second average diameter to the first average Diameter is 5 to 500. Method according to one of claims 1 to 13, wherein the membrane a flow rate from 25 cm / min at 10 psid (68.95 kPad) to 500 cm / min at 10 psid (68.95 kPad). Method according to one of claims 1 to 14, wherein a Thickness of the membrane is less than 140 microns. Method according to one of claims 1 to 15, wherein a Thickness of the membrane is less than 75 microns. Method according to one of claims 1 to 16, wherein a Thickness of the membrane is less than 70 microns. Method according to one of claims 1 to 17, wherein a Thickness of the membrane is less than 30 microns.